Development of solid polymer fuel cells that employ liquid fuel is being actively promoted for use as a power source for various electronic apparatuses including mobile phones, because such cells are easy to be made smaller in dimensions and lighter in weight.
The solid polymer fuel cell includes a Membrane and Electrode Assembly (MEA), in which a solid polymer electrolytic membrane is interleaved between an anode and a cathode. The fuel cell that directly supplies the liquid fuel to the anode is called a direct-type fuel cell, in which the supplied liquid fuel is decomposed on a catalyst carried by the anode, so that positive ion, electron and an intermediate product are given. In the fuel cell of this type, the positive ion thus generated further migrates to the cathode through the solid polymer electrolytic membrane, while the generated electron migrates to the cathode through an external load, to be reacted with oxygen in the atmosphere on the cathode, thereby generating electricity. In the direct methanol fuel cell (hereinafter, DMFC) that employs, for example, methanol aqueous solution as it is as the liquid fuel, the reaction represented by the formula (1) cited below takes place on the anode, and the reaction represented by the formula (2) cited below takes place on the cathode. As is apparent from these formulae (1) and (2), theoretically 1 mol of methanol and 1 mol of water are reacted on the anode to thereby give 1 mol of reaction product (carbon dioxide) on the DMFC, and since hydrogen ion and electron are also generated simultaneously, the theoretical concentration of methanol in the methanol aqueous solution, serving as the fuel, is approx. 70% in volume (vol. %).CH3OH+H2O→CO2+6H++6e−  (1)6H++6e−+3/2O2→3H2O  (2)
It is known, however, that in the case where a relatively larger amount of alcohol fuel is supplied to the anode with respect to water, what is known as “cross-over effect” takes place in which the alcohol fuel is transmitted through the solid polymer electrolytic membrane without being involved in the reaction represented by the formula (1) cited above, to be reacted with the catalyst on the cathode, which results in decreased generation capacity and generation efficiency.
Techniques developed to suppress the cross-over effect include, for example as described in the Patent document 1, providing a porous material or the like that vaporizes the liquid fuel on the upstream side of the anode of the MEA, to thereby supply the vaporized liquid fuel. The Patent document 1 states the advantage thereof, for example as “Supplying thus the vaporized fuel allows maintaining the gas fuel in the fuel vaporization layer substantially saturated, the liquid fuel is vaporized in the amount corresponding to the consumption of the gas fuel in the fuel vaporization layer for the cell reaction, and then the liquid fuel of the amount corresponding to the vaporized amount is introduced into the cell via capillary effect. Thus, since the fuel supply amount is linked with the fuel consumption, the fuel is scarcely discharged unreacted out of the cell, which minimizes the need to provide a processing system on the fuel outlet side.”
According to the Patent document 1, however, the fuel is supplied by pressure from the fuel source or a capillary effect or the like, which incurs the disadvantage that when the CO2 gas generated on the anode resides between the anode and the gas-liquid separation membrane, the pressure against the liquid fuel source is increased and the fuel supply to the anode is thereby suppressed, and consequently the generation performance becomes unstable.
To resolve such problem, a fuel cell and a fuel cell system configured so as to efficiently discharge the generated CO2 gas have been developed, as shown in FIG. 3.
According to such invention, the fuel cell includes a discharging device that discharges the product generated through the electric reaction on the anode (predominantly CO2), and the discharging device is a ventilation port provided in a sealing material interleaved between the solid polymer electrolytic membrane the collecting electrode of the anode, and therefore the CO2 can be discharged in a single direction from a lateral side of the anode, while the vaporized fuel is being supplied. Consequently, the CO2 gas generated on the anode is prevented from residing between the anode and the fuel supply controller, so that the increase in pressure against the fuel source is prevented and sufficient fuel supply to the anode can be secured. Thus, the fuel cell incorporated with the technique as shown in FIG. 3 provides higher fuel consumption efficiency, and also higher stability in generating performance over a long period of time.
With the technique as shown in FIG. 3, however, there may be cases where degradation of the MEA is not sufficiently prevented. For example, if the fuel cell is left unused for a long time after generation, a portion of the anode and the anode-side collecting electrode close to the CO2 gas outlet is prone to corrode. Such corrosion can be construed to take place through the following mechanism. First, the residual alcohol fuel on the anode is reacted, via the anode catalyst metal, with air introduced through the ventilation port lateral to the anode for discharging the CO2 gas, and thereby water is generated in the vicinity of the ventilation port. Here, the ventilation port is constituted of the collecting electrode of a metal which is hydrophilic, and hence the water is encouraged to reside in its vicinity. Thus, the fuel density at the portion of the anode close to the ventilation port becomes locally uneven, and a partial cell is thereby formed. Consequently, the material constituting the anode and the collecting electrode is urged to corrode, which makes it difficult to sufficiently prevent the degradation.
To sufficiently suppress the degradation of the MEA, the water has to be kept from residing in the ventilation port for discharging the CO2 gas in the vicinity of the anode.    [Patent document 1] JP-A No. 2000-106201